Hartford - Republicans stepped up pressure Monday on Democratic Gov. Dannel P. Malloy and the General Assembly's majority Democrats to stop a looming increase in the state's gasoline and diesel taxes, launching a petition drive at gas stations across Connecticut and on the Internet.

GOP leaders said the higher taxes, originally approved years ago and set to take effect July 1, will hurt middle-class families and struggling businesses. They also criticized the Democrats for using the revenue to help balance the state budget and not pay for road and bridge repair.

"There's still time to stop this unfair tax increase," said Senate Minority Leader John McKinney, R-Fairfield, standing across from a Hartford gas station with House Minority Leader Lawrence Cafero Jr., R-Norwalk. Both men are considering running for governor in 2014.

The petroleum gross receipts tax is a percentage of the wholesale price of gasoline and is charged to companies distributing petroleum products in Connecticut. It is scheduled to increase from 7 percent to 8.1 percent, raising prices by about 4 cents per gallon. The tax would be in addition to the 25-cents-per-gallon state tax on gas and the 18.4-cents-per-gallon in federal gas taxes.

Democrats took issue with the Republican criticism.

"In 2005, Senator McKinney and Representative Cafero both voted in favor of the bill that scheduled this change, and it was signed into law by a Republican governor," said Andrew Doba, Malloy's spokesman. "Can someone please notify the credibility police?"

Adam Joseph, a spokesman for the Senate Democrats, accused the GOP leaders of playing politics and criticized them for not producing an alternative to the budget passed by the Democrats.

"It's not even the dog days of summer and the Republican leaders are howling about a tax they led the charge to raise," he said.

Cafero said 2005 "was a very different time" with a better economy and lower unemployment. Since then, the Republicans have taken credit for helping to scuttle a previous scheduled increase and for imposing a one-time cap on petroleum gross receipts tax.

The hike next month would mark the third in three years for the state's diesel tax, however, said Michael Riley, president of the Motor Transport Association of Connecticut. It is scheduled to climb by 3.5 cents per gallon.

"The two biggest expenses a trucking company has are labor and fuel. And this is going to be very difficult for some of my guys to contend with," he said, adding that a number of his members have gone out of business over the last several years, including a 100-year-old trucking company based in Hartford that closed its doors three weeks ago.

Estimating that an average truck can get 5 miles per gallon, Riley said the tax hike will boost the overall levy to 55 cents per gallon, or 11 cents per mile. He said every big truck that travels through Connecticut will pay that tax under a federal tax collection system.

Riley is particularly upset that not all the revenue will be deposited in the state's Special Transportation Fund to pay for roads and bridges.

"It's just not necessary to bang the daylights out of a critically important industry like this in order to fund general revenues," he said. "We need roads and bridges to be fixed, and we don't need to be cranking up the fuel tax to pay for general fund expenses."